1. Field of the Invention
The present invention relates to a process for manufacturing a silicon-on-insulator substrate having a structure in which an oxide layer is locally buried within a silicon substrate.
Priority is claimed on Japanese Patent Application No. 2005-290705, filed Oct. 4, 2005, the content of which is incorporated herein by reference.
2. Description of Related Art
Silicon-on-insulator (SOI) substrates in which an oxide layer, i.e., an insulator layer made of oxide known as a buried oxide (BOX) layer, has been buried at the interior of a silicon substrate by the local implantation of oxygen ions in a single-crystal silicon substrate and annealing treatment involving the application of a given amount of heat have attracted attention in recent years as semiconductor substrates for high-performance transistors. One known application for such SOI substrates is the partial SOI substrate in which a region corresponding to the BOX layer (referred to below as the “SOI region”) is locally formed at the interior of the substrate. The partial SOI substrate is of value in system LSIs having integrated thereon, for example, analog, logic and memory functionality because such a substrate makes it possible to form only logic circuits in the SOI region and to form memory circuits in other regions (referred to below as the “bulk regions”).
However, when a substrate having such a SOI structure is manufactured, there is a risk that, in the course of forming the BOX layer in annealing treatment, the volume of the BOX layer will expand, causing the surface of the substrate to swell and create a “step.” When semiconductor devices are formed on the surface of a substrate having such steps thereon, problems such as a loss of focus may arise in the photolithography process.
One solution, described in JP-A 2004-193185, is a process for manufacturing a SOI substrate in which ions are implanted to the SOI region after portions of the silicon substrate surface, i.e., bulk regions, have been masked with a surface oxide film, following which the thickness of the surface oxide film is reduced and annealing is carried out in an oxidizing atmosphere, thereby forming a BOX layer within the substrate. That is, during annealing treatment, a surface oxide layer forms on the surface of the substrate in the SOI region, and this layer is lifted up on the surface side with formation of the BOX layer at the interior. However, the presence of an oxide film of a given thickness within the bulk regions alleviates the size of the resulting step. In this way, by removing the oxide film or surface oxide layer in a post-annealing operation, a relatively flat substrate surface can be achieved.
Yet, in silicon substrates obtained by such a method of manufacture, as noted above, when a BOX layer is formed, the BOX layer expands and the surface of the substrate swells, while at the same time oxygen diffuses and oxidation proceeds at the surface of the substrate, forming a surface oxide layer. At edge areas of the BOX layer in particular, oxidation is accelerated by the diffusion also of oxygen from the bulk regions, thus increasing the thickness of the film in the edge areas of the BOX layer. Depending on the degree of surface oxide layer formation, the thicker edge areas of the BOX layer may reach the surface of the substrate and connect to the surface oxide layer. Removal of the surface oxide layer may cause the BOX layer which projects out to the substrate surface being overetched together with the surface oxide layer, leading to the formation of cavities at the substrate surface, which may have detrimental effects such as particle formation during device fabrication.
It is therefore an object of the present invention to form an oxide layer, i.e., a BOX layer, at the interior of the silicon substrate while preventing any thickening of the BOX layer that may allow edge areas of the BOX layer to reach the substrate surface.